http://www.theregister.co.uk/2011/07/21/mac_os_x_lion_security/
With Wednesday's release of Mac OS X Lion, Apple has definitively leapfrogged its rivals by offering an operating system with state-of-the-art security protections that make it more resistant to malware exploits and other hack attacks, two researchers say.
Unlike the introduction of Snow Leopard in 2009, which offered mostly incremental security enhancements, OS X 10.7 represents a major overhaul, said the researchers, who spent the past few months analyzing the OS.
“It's a significant improvement, and the best way that I've described the level of security in Lion is that it's Windows 7, plus, plus,” said Dino Dai Zovi, principal of security consultancy Trail of Bits and the coauthor of The Mac Hacker's Handbook. “I generally tell Mac users that if they care about security, they should upgrade to Lion sooner rather than later, and the same goes for Windows users, too.”